With a recent development of digital technology and improvement in data compression technology, optical discs such as DVDs (digital versatile discs) have been attracting attention as information recording media for storing computer programs, music information, and video information (hereinafter also referred to as “contents”). As the price of such optical discs drops, optical disc apparatuses for reproducing information recorded on the optical discs are now becoming widely used.
The amount of content information tends to increase year by year, which leads to an expectation for a further increase in the storage capacity of optical disks. As a means to increase the storage capacity of an optical disk, extensive efforts are being made with respect to the development of optical disks having a plurality of recording layers and also with respect to the development of optical disc apparatuses for accessing such optical disks.
As a read-only optical disk having two recording layers, a DVD-ROM having two recording layers on one side thereof (hereinafter referred to as “single-side two-layer DVD-ROM”) has been put into practical use. With this single-side two-layer DVD-ROM, a laser beam is shone from one side, and the focus of the optical beam is placed on the individual layers to perform reproduction separately for each recording layer. This thus allows the information recorded on each recording layer to be reproduced without flipping over the disc. Further, as an optical disc having two recording layers capable of storing information, a DVD-R having two recording layers on one side thereof (hereinafter referred to as “single-side two-layer DVD-R”) is known. With this single-side two-layer DVD-R, it is possible to record information on each recording layer and reproduce information recorded on each recording layer without flipping over the disc. In the following, an optical disc having two recording layers (referred to as LAYER-0 and LAYER-1) on one side thereof will be referred to as a single-side two-layer disc.
In information reproducing apparatuses adapted to the single-side two-layer disc, one of the two scan paths (scan path A and scan path B) is employed as a scan path when scanning an optical spot from an address (current address) on one recording layer (referred to as X) to an address (target address) on the other recording layer (referred to as Y). In the scan path A, the focus is jumped from the recording layer X to the recording layer Y at the current address position, followed by performing a seek operation toward the target address on the recording layer Y. In the scan path B, an address on the recording layer X at the same position as the target address in terms of the disc radial direction is set as a tentative target address, and a seek operation is performed from the current address to the tentative target address, followed by jumping the focus at the tentative target address position from the recording layer X to the recording layer Y (see Patent Document 1, for example). In general, the scan path A is easier than the scan path B in terms of achieving high-speed scan, and is thus employed in many information reproducing apparatuses adapted to the single-side two-layer disc. Further, there are apparatuses that perform focus jump and seek operation at the same time (see Patent Document 2, for example).
As shown in FIG. 12A, all the data area of a layer LAYER-0 is a prerecorded area, and a portion (on the inner circumference side) of the data area of a layer LAYER-1 is an unrecorded area. In such a single-side two-layer disc (e.g., a single-side two-layer DVD+R prior to finalizing), an optical spot may be scanned from an address a (current address) on the inner circumference side of the layer LAYER-0 to an address b (target address) in the prerecorded area of the layer LAYER-1. In such a case, focus jump is performed at the position of the address a to the layer LAYER-1 as shown in FIG. 12B when the scan path A is employed. In this example, the layer LAYER-1 has an unrecorded area at the same position as the address a in the disc radial direction, so that address information included in reproduced data cannot be acquired. There is thus a risk of failing to perform a proper focus jump.
When the optical spot is scanned from the address b (current address) to the address a (target address) as shown in FIG. 12C, a tentative target address a′ is set, followed by performing a seek operation to the tentative target address a′ on the layer LAYER-1 when the scan path B is employed. In this case, however, the tentative target address a′ is situated in the unrecorded area. There may thus be a risk of suffering a seek error without being able to acquire address information contained in reproduced data.
As described above, when an unrecorded area is coexistent with a prerecorded area, an information reproducing apparatus adapted to a single-side two-layer disc may fail to reproduce information recorded on a target address even though the target address is situated in the prerecorded area.
[Patent Document 1] Japanese Patent Application Publication No. 9-282675
[Patent Document 2] Japanese Patent Application Publication No. 2002-8252
Accordingly, there is a need for a reproduction method and optical disc apparatus that can properly reproduce information in a stable manner from an optical disc having a plurality of recording layers in which a prerecorded area and an unrecorded area are coexistent.
There is another need for a program performed by the control-purpose computer of an optical disc apparatus and a recording medium having such a program embodied therein which can reproduce information in a stable manner from an optical disc having a plurality of recording layers in which a prerecorded area and an unrecorded area are coexistent.